Under the Salazar regime, although the constitution did not prohibit political activity, the National Union (União Nacional) was the only political party represented in the legislature. Candidates of the old Center parties, which had been active prior to 1926–28, were allowed to participate in national elections starting in 1932, although none were ever elected.
After the 1974 revolution, several right-wing parties were banned, and various left-wing parties that had functioned underground or in exile were recognized. Among these was the Portuguese Communist Party (Partido Comunista Português—PCP), which was founded in 1921 and is Portugal's oldest political party. It is especially strong among industrial workers and southern farm workers. The government also recognized the Portuguese Socialist Party (Partido Socialista Português—PSP), founded in exile in 1973, and the Popular Democratic Party (Partido Popular Democrático—PPD), formed during the Caetano regime; both the PSP and the PPD favored the establishment of a Western European-style social democracy. Tied to the policies of the Caetano regime was the Social Democratic Center (Centro Democrático Social—CDS), founded in 1974, which held its first conference in January 1975 and became a target for left-wing disruptions. In June 1976, Gen. António Ramalho Eanes, the army chief of staff, who was supported by the major non-Communist parties, won election as Portugal's first president.
In 1979, the right-of-center Democratic Alliance (Aliança Democrática—AD) was formed by the Social Democratic Party (Partido Social Democrático—PSD), founded in 1974; the CDS; and the People's Monarchist Party (Partido Popular Monárquico—PPM). The leftist United People's Alliance (Aliança Povo Unido), also formed in 1979, included the People's Democratic Movement (Movimento Democrático Popular—MDP), dating from 1969, and the PCP.
The Republican and Socialist Front (Frente Republicana e Socialista—FRS), formed in 1980, consisted of the PSP, the Union of the Socialist and Democratic Left (União da Esquerda Socialista Democrática—UESD), founded in 1978, and Social Democratic Independent Action (Acção Social Democrata Independente—ASDI), founded in 1980. The People's Democratic Union (União Democrática Popular—UDP), dating from 1974, comprised political groups of the revolutionary left.
In October 1985, former President Eanes's centrist Democratic Renewal Party (Partido Renovador Democrático—PRD) entered the ballot for the first time, taking 18% of the vote. In 1991, the seats were distributed as follows: PSO, 135; PSP, 72; CDU, 17; Center Democrats, 5; National Solidarity, 1. The latter was formed in 1990 to address the needs of pensioners.
In the legislative elections of 10 October 1999, the seats were distributed as follows: PS, 114; PSD, 83; CDU, 17; CDS/PP, 15, and Left Bloc, 2. Antonio Manuel de Oliviera Guterres was reappointed prime minister. The first PS victory in October 1995 carried through to the presidential elections of 14 January 1996, when Jorge Sampaio was elected president, with 53.8% of the vote to Aníbal Silva's 46.2%.
Guterres ruled during a downturn in the global economy in his second term, and in December 2001 he resigned following a defeat for the PS in municipal elections. The PSD, led by José Manuel Durão Barroso, won 40.1% of the vote and took 105 seats in parliament in the 17 March 2002 elections, to the PS's37.9% and 96 seats. The Popular Party won 8.8% of the vote and secured 14 seats; the PSD formed a coalition government with the PP. Also winning seats were the CDU (Unitarian Democratic Coalition, comprised of the Portuguese Communist Party and the Greens), 7% of the vote and 12 seats; and the Left Bloc (comprised of the communist Democratic People's Union, the Revolutionary Socialist Party, and the extreme left party Politics XXI), 2.8% of the vote and 3 seats.